Rotating components of machinery, such as shafts, are continuously subject to torsional forces and resulting torsional vibrations. Though there is a wide variety of machinery that uses rotating shafts, power generators are particularly dependant on properly functioning rotor shafts. It is imperative that the torsional motion is measured accurately for variances and fluctuations that are indicative of improper tuning of torsional modes or excessive torsional forces. The sooner that such torsional vibrations are detected, the more reliably the mechanical operations can run, and the less damage the rotating components will cause to themselves and surrounding parts.
Torsional forces will create problems such as uneven rotation, which the optical probe 2 registers as a non-uniform reading of the pattern 8. The greater the sensitivity of the system, the sooner problems in the rotating components can be detected and remedied. As such, it is of paramount importance that the pattern be as uniform as possible. Even slight variances in the pattern can create large background readings that negate the higher levels of sensitivity of the entire system.
Usually when torsional forces are being measured, the normal operations of the related machinery need to be shut down. In the case of power generators, every hour of shutdown is a considerable loss in revenue and efficiency, sometimes totaling thousands of dollars per hour. Therefore it is important to minimize the length of time that a rotor is not operating. This is particularly important during forced outages, where there is a problem in the machinery that needs to be pinpointed and corrected quickly. However, it is also important to minimize the down time during scheduled outages, such as those for maintenance.
Different techniques have been developed for monitoring the torsional response of rotating components. One technique is for an optical probe to scan a circumferential area around a rotating shaft, as shown in FIG. 1. In this example, a fiber optic probe 2 is mounted to a stationary fixture 4 such as a bearing bracket, and optically scans a circumferential area on a rotating shaft 6. The area scanned needs to have some form of optically identifiable pattern 8, such as alternating bright and dark bands. The pattern is uniform around the circumferential area so that the optical probe reads a continuously repeating pattern as the shaft rotates. An example of such an approach is disclosed in U.K. Patent Application GB2093991A.
When utilizing optical torsional techniques like those described above, it is imperative that the optical pattern be evenly and precisely distributed about the circumference of the component being monitored. If the bright/dark bands are not evenly spaced and distributed, then there will be a variation in sensed frequency as the uneven bands rotate past the optical probe. These frequency variations will produce a signal which will appear to be similar to an actual torsional signal, but will instead be a false signal or noise as a result of the uneven spacing.
This is a particular challenge when patterns are retro-fitted onto rotating components after they are built and in use for a while. This creates problems in placing a uniform pattern onto a round surface and making sure that each pattern segment has the same width and spacing as all of the rest.
In order to create a uniform pattern, it is common practice in the art to provide a striped pattern strip, where the stripes have been created by computer or some other sort of accurate printing means, typically in the form of an adhesive tape or template for spray painting. Though this may provide for a uniform pattern along the length of the strip, it is unfortunately the case that despite best efforts, the ends of the strip almost never align properly. Therefore, where the ends of the strip join, the optical probe will read a non-uniform segment pattern. Depending on the error in joining the ends of the patterned strip, this can create a large background error that will effectively bury small variations in the torsional vibrations.
What is needed is a method for placing a uniform pattern onto a rotatable component such that all of the pattern segments are uniformly spaced. This needs to be done in a quick and accurate manner to minimize downtime while maintaining the pattern quality.